A bellows is a spring-like member which may be used in a great many instruments such as pressure responsive valves. For example, gas-lift valves which are used in the oil and gas industry in the production of oil often include a bellows member such as disclosed by U.S. Pat. Nos. 3,175,154 and 3,530,566 of McMurry Oil Tools, Inc. As used in such valves, the bellows is typically a hollow cylindrical device constructed of a flexible material, typically thin metal, and having plurality of circumferential corrugations in the form of a plurality of inwardly opening convolutions and a plurality of outwardly opening convolutions which are arranged in alternating relation along the axial length of the bellows. The bellows expands and contracts along its longitudinal axis in response to pressure differential, thereby causing expansion and contraction of each of the convolution along the length of the bellows and providing for bellows urged movement of an internal component such as a valve member while the bellows maintains a sealed, pressure containing partition within the valve assembly.
In the past, to solve problems of the bellows becoming weakened along fold lines defined generally by the convolutions as the convolutions are subjected to excessive pressure differential, the bellows construction has been provided with internal and external back-up rings within the convolutions which provide structural support and thus minimize excessive deformation of the bellows. The subject matter of U.S. Pat. No. 2,963,043 of Davis, et al, discloses back-up rings in the form of split rings which are located within the internal bellows convolutions. Generally, however, it has been determined that a more satisfactory bellows construction may be achieved through the provision of shrink fit type polymer back-up rings which are disclosed in U.S. Pat. Nos. 3,530,566 and 3,530,770 of McMurry, et al.
When the typically metal bellows construction is secured at respective ends thereof to corresponding fixed and moveable gas-lift valve components, such attachment is typically accomplished by means of soldering. When this is done, a small end section of the bellows, referred to herein as the heat affected zone, is subject to sufficient heating that the polymer back-up rings of the bellows construction can melt or sublimate to the point that the bellows convolutions may not be provided with proper support, especially at the end portions thereof. According to the McMurry '566 and '770 Patents, identified above, special heat-resistent rings are used for inwardly-facing or opening convolutions adjacent the ends of the bellows where soldering or other heating operations may occur. These heat-resistent back-up rings are intended to withstand the heat of soldering in the heat affected zones without in any way changing the configuration thereof. Typically these heat-resistent back-up rings are composed of asbestos while other back-up rings of the bellows construction are composed of any one of a number of suitable polymer materials such as polytetrafluoroethylene which is sold under the registered trademark, TEFLON.
In more recent times, gas-lift induced production activities are being accomplished at deeper production zones where greater gas pressures and hydrostatic pressures are likely to be encountered. Thus, the bellows construction of gas-lift valves must be more resistent to higher pressure differentials than in the past. The respective ends of the bellows, to withstand high pressure differentials, may be connected and sealed to the respective fixed and moveable internal valve components by means of high temperature solder or by welding operations wherein substantially greater heat is applied to the ends of the bellows than in the past, thereby causing the heat effective zones of the bellows to be significantly larger. In the event polymer back-up rings are employed in the bellows convolutions, a significantly greater number of them would be typically damaged by the heat of high temperature soldering or welding. When this happens the bellows convolutions will not be properly supported and pressure induced damage can occur. It is desirable, therefore, to provide a suitable construction and method of manufacturing for the bellows assembly of gas-lift valves to permit high temperature soldering or welding without consequent heat induced damage to the back-up rings of the bellows convolutions.